The present invention relates to fluidized bed combustors wherein a particulate fuel is combusted in a fluidized state, and, more particularly, to such a combustor having an integral solids separator for removing particulate material carried over in the flue gas generated in the combustion chamber.
In a typical present day fluidized bed combustor, particulate fuel, such as coal having a top size ranging up to about 6.5 millimeters, is typically fed to and combusted within a furnace chamber in a fluidized state in a fluidizing gas at relatively low temperatures ranging from about 760.degree. C. to 925.degree. C. Fluidized bed furnaces are particularly adaptable to burning sulfur containing fuels as the particulate material fluidized within the furnace may include a sulfur absorbent, most commonly crushed limestone, in addition to the particulate fuel. Fluidizing air, which also serves as combustion air, is supplied to the furnace from an air plenum located beneath the combustion chamber. In a typical circulating fluidized bed, also referred to as a fast fluidized bed, the velocity or flow rate of the fluidizing air being passed upwardly into the furnace is maintained at a level sufficiently high to entrain most of the particulate material present within the furnace such that a substantial portion of the particulate material within the combustion chamber is carried therefrom with the flue gas.
Accordingly, it is necessary on circulating or fast fluidized bed combustors to provide a means for removing the particulate material carried over from the combustion chamber with the flue gas prior to venting the flue gas to the atmosphere. As this particulate material typically contains a significant amount of unburned fuel, it is further advantageous to collect the particulate material from the gas stream for recycle to the combustion chamber of the furnace for further combustion in order to increase the efficiency of fuel utilization. Additionally, the particulate material removed from the gas stream will generally contain a significant amount of unreacted sulfur absorbent which is also recirculated to the combustion chamber of the furnace in order to increase the efficiency of utilization of the sulfur absorbent.
The most common means for separating the particulate solids from the flue gas passing from the combustion chamber prior to venting same to the atmosphere is a cyclone separator. A circulating fluidized bed system utilizing a cyclone for a particulate collection is illustrated in U.S. Pat. No. 4,111,158. As shown, the cyclone is typically installed immediately downstream of the furnace in order to facilitate recycle of the collected particulate material to the furnace. As the particulate solids entrained in the flue gas will have a temperature reflecting the temperature of the flue gas, the cyclone separator will be exposed to both hot solids and hot gases and therefore must be designed to withstand temperatures ranging as high as 1600.degree. C. under normal operation. Additionally, the cyclone separator must be designed to survive operation in a highly erosive environment as the particulate loading in the flue gas will result in a significant impact of erosive particles on the side walls of the cyclone. Accordingly, the type of cyclone suitable for utilization in a circulating fluidized bed system has a high capital cost and frequently a high operating cost associated therewith.
It would be desirable to substitute a less expensive separating means for the cyclone on a typical circulating fluidized bed furnace. Examples of such circulating fluidized bed furnace systems utilizing non-cyclonic separators are illustrated in U.S. Pat. Nos. 4,442,797 and 4,538,549 which show, respectively, the use of momentum separators and the use of impact separators for removing particulate material from the flue gas passing from the combustion chamber.
In U.S. Pat. Nos. 4,442,797, the flue gas passing from the furnace chamber to the flue gas vent duct must exit the furnace chamber through a plurality of vertical slot-like openings formed by bending the waterwall tubes in the upper wall of the combustion chamber inwardly from the plane of the wall to provide a flow area between adjacent tubes. A plenum chamber extends in belt-like fashion around the upper region of the furnace chamber to receive the flue gas which passes through these tubes. As the flue gas passes through the slot-like openings formed between tubes, the flue gas drops in velocity below the entrainment level and the particles carried therein drop out of the flue gas stream and are directed to a collection hopper for recycle to the furnace chamber.
In U.S. Pat. No. 4,538,549, impact beams are provided at and immediately downstream of the gas outlet of the furnace chamber. The flue gas leaving the combustion chamber of the furnace passes between the rows of impact beams while the particulate solids contained therein, due to their flow inertia, strike the impact beams. As a consequence of striking the impact beams, the particles lose their momentum and drop out of the gas stream into a collection hopper for recycle to the furnace while the flue gas passes on to the vent stack.
It is an object of the present invention to provide a fluidized bed combustor having an integral non-cyclonic solids separator for removing the hot particulate solids carried in a flue gas stream prior to venting the flue gas stream to the atmosphere.